Can work in one database affect another?

While logged into a database, working exclusively through the web interface, with all technical features enabled and even going into debug mode is it possible to make a change that will affect another database in any way? In particular, all the settings.

We go to a duplicate of the production database when we want to perform tests such as new modules or trying out some unused OpenERP features. What areas should be off limit if a production database is also on the same server as the test database?

For example, there are some aspects of OpenERP that are not stored in Postgresql. I'm sure Postgresql has mechanisms to segregate databases. But, aren't there files in the OS filesystem that are shared by all the databases?